The reason why a sauna at 100°C (212°F) is not harmful while water boiling at the same temperature can be harmful is primarily due to the difference in the nature of heat transfer and the way our bodies interact with each.
In a sauna, the temperature refers to the air temperature, not the temperature of objects within the sauna. The air in a sauna can be heated to 100°C, but the surfaces and objects in the sauna, such as the walls and benches, are not at the same temperature. When you enter a sauna, your body absorbs heat primarily through convection and conduction from the surrounding air and surfaces. The air temperature in the sauna can feel hot, but your body temperature does not rise significantly because the heat is not being directly transferred to your body's core. The high humidity in the sauna also helps to dissipate heat through evaporation of sweat, further aiding in cooling your body.
On the other hand, when water boils at 100°C, it undergoes a phase change from liquid to vapor (steam). The process of boiling requires the addition of a substantial amount of heat energy, known as the latent heat of vaporization. When boiling water comes into contact with your skin, it can transfer heat rapidly and cause burns. The conversion of water into steam releases a significant amount of energy, which can scald the skin and cause severe damage.
In summary, the difference lies in the mode of heat transfer and the nature of the substances involved. In a sauna, heat is primarily transferred through convection and conduction, while in boiling water, the heat is transferred through direct contact and the release of latent heat during the phase change.